1. Field of the Invention
The present invention relates to an apparatus for producing single crystals, such as single crystal silicon ingots, grown by the Czochralski (CZ) method also known as the pulling method. More particularly, it relates to such an apparatus which is capable of producing a large-diameter, heavy Czochralski-grown single crystal with safety.
2. Description of the Prior Art
In the production of single crystals, such as single crystal silicons, grown by the Czochralski (CZ) method, polycrystalline silicon powder is melted down within a quartz crucible heated in a chamber by resistance heating or high frequency heating. After a seed crystal is attached to the lower end of a pull shaft or a pull wire disposed above the quartz crucible, the pull shaft or the pull wire is covered to dip or immerse the seed crystal into a melt of polycrystalline silicon. Subsequently, the seed crystal is slowly pulled up from the silicon melt while rotating at a predetermined speed, thus producing a single crystal silicon ingot grown by the CZ method.
The conventional single crystal ingot thus produced has a diameter of 5-6 inches (12.55-15.24 cm) and a weight of 20-30 kg. In recent years, with a view to improving the yield of the grown single crystal and the production efficiency of semiconductors from the crystal, there has been a growing tendency toward producing a single crystal ingot which is larger in diameter (more than 7 inches (17.78 cm), for example) and longer in length than conventional single crystal ingots. The weight of such larger-diameter and longer single crystal ingots sometimes exceeds 100 kg.
When a single crystal ingot is being pulled, the weight of the single crystal ingot is sustained by a seed crystal ingot and a small-diameter Dash's neck contiguous to the seed crystal. Since the Dash's neck is only 1-4 mm in diameter, it has a limited capability of withstanding a load exerted thereon. Consequently, the Dash's neck is likely to be broken when subjected to a small twisting force or an impact while the single crystal ingot is being grown or during the handling operation of the single crystal ingot. A break in the Dash's neck would cause various accidents such the destruction of a single crystal producing apparatus, the flooding of a polycrystalline melt, or a phreatic explosion which might result in injury or death.
In order to avert danger, an attempts have been made to use large-diameter head and small-diameter neck formed in succession on an upper part of a large-diameter, heavy single crystal ingots being grown so as to provide a downwardly facing engagement step. The stepped upper part of the single crystal ingot being grown is gripped by a plurality of clamp arms (grip means) each having a hook-shaped clamp finger lockingly engageable with the engagement step or by a plurality of vertically movable shafts (grip means) each having a locking projection engageable with the engagement step. Thus, the weight of the large-diameter, heavy single crystal ingot being grown is sustained by the stepped upper part which is considerably larger in diameter than the Dash's neck and hence is capable of withstanding by far a larger load than the Dash's neck. The foregoing attempt is disclosed in Japanese Patent Laid-open Publication Nos. 55-167200, 62-288191 and 63-252991.
The foregoing attempt is not satisfactory because the grip means fails to hold the engagement step in a stable and reliable manner. More specifically, when a very heavy single crystal ingot having a weight over 100 kg is being pulled, the grip means yields to the weight of the single crystal ingot and tends to spread radially and outwardly with the result that the single crystal ingot detaches from the grip means.
Furthermore, the stepped upper portion of the single crystal ingot including the engagement step is sometimes formed asymmetrically about a longitudinal axis of the single crystal ingot. Such an asymmetric engagement step cannot be held stably and reliably by the grip means, and due to unequal sharing of the weight of the single crystal ingot among the grip means, the single crystal detaches from the grip means. In this instance, a dangerous accident such as destruction of the single crystal ingot producing apparatus may occur.